1. Field of the Invention
The present invention relates to an EHA (Electro Hydrostatic Actuator) constituted by an integral combination of a motor, a pump, an actuator and a controller. More particularly, the present invention is concerned with an improvement of a reservoir built-in type actuator capable of attaining reduction in both size and weight of the device and capable of being installed particularly in an aircraft involving strict restriction on space.
2. Description of the Related Art
Conventional aircraft of medium or larger size are equipped with a hydraulic supply system and, by utilizing the hydraulic force, there are performed operation of rudder surfaces, upping and downing of a landing gear, and braking and leg steering operations. There is a recent tendency to a total electric drive, eliminating the conventional hydraulic source driven by shaft output of an engine and for improving the fuel efficiency.
If it is possible to abolish the hydraulic supply system installed in aircrafts and each actuator can perform a self-completed type operation with electric power, not only the fuel efficiency is improved, but also such a flight system is noted as a reliable next-generation flight system. For example, in the conventional hydraulic supply system, it is always necessary to continue supply of a high pressure fluid irrespective of whether an actuator is in operation or not. However, according to the above flight system, it suffices to supply electric power only when the operation of an actuator is needed. Thus, not only the flight system in question can contribute to the saving of energy in the aspect of application, but also such a latent risk factor as a hydraulic piping of a high pressure can be eliminated.
As such actuators, two types of actuators are in practical use at present. One is EHA and the other is EMA (Electro Mechanical Actuator). In the EHA, a hydraulic system is present in the interior of actuator and a motor for a hydraulic pump to operate the hydraulic system is supplied, with power by electricity from the exterior. On the other hand, EMA is a structure using an electric motor for operation, doing away with a hydraulic system completely.
A comparison between EHA and EMA shows that EMA is simpler in structure because a hydraulic system is not installed therein and that therefore EMA is considered more advantageous for attaining reduction of both size and weight. However, in actuators for use in aircrafts where high reliability is required, EHA is adopted in many cases. Thus, with EMA, it is difficult to ensure high reliability at the present technological level. For this reason, EHA is mainly adopted (see, for example, Japanese Patent Application Publication No. 2002-054604).
For actuators used in aircrafts, various studies are being made because there exists a strong demand for attaining reduction of both size and weight. For example, in conventional actuators, a motor, a pump and a reservoir mechanism are disposed as external components, thus obstructing the reduction in size and weight of an EHA device. For solving such a problem, the applicant in the present case proposes in Japanese Patent Application Publication No. 2006-105364 a so-called reservoir built-in type actuator as an actuator attaining the reduction in size of the entire EHA device, the actuator incorporating a reservoir into a piston rod comprising a hollow structure, the reservoir having been installed as an external component in the conventional EHA structure.